The present invention relates to a thermal displacement correction method for a machine tool, and an apparatus for implementing the method.
In a machine tool, a feed screw and a main spindle are driven by means of a motor. Therefore, the main spindle and the feed screw are expanded by heat from the motor, frictional heat from rotating bearings, and frictional heat from an engaging portion between a ball screw and a ball nut of the feed screw, causing the displacement of the machine position. That is, the relative positions of a workpiece to be positioned and a tool are displaced. The change in machine position that is attributed to heat gives rise to problems in high-accuracy machining.
Conventionally, to prevent the thermal displacement of the machine, methods such as the following have been employed.
(a) providing a cooling device and applying initial tension to the ball screw of the feed screw, thereby providing a structure that is free from influences of thermal expansion, or
(b) providing a displacement sensor and a temperature sensor so that commanded positions can be corrected in accordance with detected displacements and detected temperatures.
The aforesaid method (a), in which the structure that is free from influences of thermal expansion is provided by applying initial tension to the ball screw of the feed screw, and the method (b), in which thermal displacement is corrected by means of the sensors, are subject to the problem of structural limits and the problem in sensor attachment positions, and require means for protecting the sensors against a coolant and cuttings to ensure reliability. In the case where the sensors are used, moreover, measurement takes a lot of time, so that the machining time is long, and a first-transition displacement cannot be corrected.
An object of the present invention is to provide a thermal displacement correction method capable of easily correcting thermal displacement at low cost without requiring use of sensors.
Another object is to provide a thermal displacement correction method capable of effecting more accurate thermal displacement correction by being used in combination with sensors. Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
In order to achieve the above objects, in a thermal displacement correction method for a machine tool controlled by means of a numerical control device according to the present invention, an approximation formula for obtaining a correction amount against a thermal displacement from the mean moving speed and moving frequency of a feed shaft and an approximation formula for obtaining a position correction amount for a commanded position from the correction amount obtained according to said approximation formula are previously stored in the numerical control device, whereby the position of the feed shaft is monitored to determine the mean moving speed and moving frequency of the feed shaft, determine the position correction amount for the commanded position of the feed shaft according to said approximation formula, and correct the commanded position by said position correction amount and output the corrected commanded position.
Preferably, an approximation formula for determining a correction amount against a thermal displacement of an axis during a power-off time of the machine tool is previously stored in the numerical control device, the time when the machine tool is disconnected from the power source and a correction amount for said time are stored in advance, and a correction amount is determined from the previously stored correction amount and the power-off time according to said approximation formula when the machine tool is reconnected to the power source and is used as a correction amount for the time of reconnection to the power source.
According to the present invention, dislocation attributable to thermal displacements of the feed shaft and a main spindle can be corrected without using sensors, so that not only costs can be reduced but also the reliability of sensors need not be taken into consideration. Further, since the thermal displacement correction can be effected at all times, the thermal displacement correction can be made even at the time of the start of machining, so that the machining accuracy can be improved.
Further, the position correction accuracy can be enhanced by being combined with a method in which thermal displacements are measured to determine correction amounts by means of sensors. In this method, the frequency of measurement by means of the sensors can be reduced, so that the overall machining time can be made shorter than in the case where thermal displacements are measured for position correction only by means of sensors.